A dock leveler is a device intended to bridge the gap between an adjacent parked vehicle at a loading dock and the loading dock itself, for the purpose of loading and unloading the vehicle. Typically, a dock leveler has a deck assembly which stores horizontally and level with the dock floor and which is pivotable about a rear hinge to allow for an angular raised position of the deck assembly. A pivoting lip is included which can extend outward from the raised deck assembly to rest on the adjacent parked vehicle which is being loaded or unloaded, thereby providing a bridge between the adjacent parked vehicle and the loading dock to allow material handling vehicles or personnel to pass between the loading dock and the adjacent parked vehicle. When not being used, the leveler is typically stored with the deck assembly in the horizontal position, and the pivoting lip hanging pendant (downwardly) and stored within lip keepers. The angular raising movement of the deck assembly away from this stored position allows clearance for the lip to pivot to its extended position. Hydraulics are typically used for the purpose of controlling the movement of both the deck and the lip, as is well-known in the art.
The present invention is directed to a hydraulic control system that uses separate hydraulic cylinders to control deck assembly motion, and the motion of the lip, as is typical. In most existing dock levelers, the operational sequence is initiated by powering a pump to supply pressurized hydraulic fluid to the deck cylinder. The deck assembly typically rises above its stored position to an upwardly inclined angle. It is typically necessary to raise the deck assembly to this inclined angle so that the lip can be pivoted from the pendant to an extended position without interference with the adjacent parked vehicle. Typically, once the deck assembly reaches a predetermined angle (i.e., the angle at which the deck cylinder is fully extended), the pressurized hydraulic fluid being provided by the pump is diverted to a second cylinder (the lip cylinder) which then pivots the lip from the pendant position to the extended position. To achieve this function, hydraulic control circuits for dock levelers usually include a sequencing valve between the main cylinder and the lip cylinder. Once the deck cylinder has extended to its full stroke, pressure in the hydraulic lines begins to rise, since the pump is still pumping and no further deck motion is possible. The sequencing valve senses this increase in hydraulic pressure and is moved from a position where it is blocking the flow of fluid to the lip cylinder to a position where it allows the flow of fluid to the lip cylinder, and extending of the lip subsequently occurs. Under this conventional operation, the lip thus automatically pivotally extends without operator intervention once the deck assembly reaches the top of its stroke.
Most existing levelers are also capable of so-called "short cycle" operation in which the lip may be extended even though the deck assembly is not raised to its full height. Such a feature gives an operator enhanced control. In a short cycle situation, the control box for the dock leveler will include a first button or other control that allows the deck assembly to be raised, and also a separate control for extending the lip (a "lipout" button). If the lip out button is pushed before the deck has risen to its full height, the lip will nonetheless extend. Typically, this is done by providing a normally open solenoid valve between the pump and the deck cylinder. Actuating the lip out control closes this normally open valve. The closing of this solenoid valve thus mimics the hydraulic pressurization that occurs when the deck cylinder is fully extended. As a result, an increase in hydraulic pressure occurs, the sequencing valve to the lip cylinder opens, and the pivoting lip can extend.
Thus, in conventional hydraulic dock leveler operation, the pivoting lip either extends automatically when the deck assembly angularly raises the top of its stroke, or it can be short cycled by mimicking the deck cylinder being fully extended, i.e., by the activation of a solenoid valve to induce the required pressure to open the sequencing valve. In certain applications, however, it may not be desirable to have the pivoting lip automatically extend when the deck assembly is at a fully raised angular position. Elimination of this automatic extension feature gives the operator total and complete control over the extend movement of the pivoting lip.